This invention relates generally to reversing nut constructions for use with diamond thread screws, and more particularly to devices of this type adapted to undergo reciprocating movement along the length of the screw, with the reversing movements in the axial direction of travel of the nut being automatically effected.
Prior diamond thread reversing screws generally employed camming grooves or shoulders having curved reversing edges at the opposite extremities of the screw. The traverse mechanisms associated with such screws typically involved elongated sliding groove followers which were contoured so as to closely follow the curvature of the grooves. Such followers were of sufficient dimension to straddle the groove intersections so that they would not jump from a left-hand to a right-hand groove or vice versa. At the ends of the screw, reversal of the nut occurred by virtue of the reversing end grooves being considerably enlarged to enable the elongated follower to swing from one groove to the oppositely directed groove.
Other types of reversing mechanisms employed fixed pins extending into the bore of a nut and riding along one of the grooves of the screw. In such devices no reliable drive means was provided for insuring continuity of travel of the pin through the groove intersections. Thus, there existed the possibility that the pin might "jump" to the oppositely directed thread groove and thus reverse the nut's axial direction of travel prematurely.
A prior automatically reversing nut construction for a diamond thread screw is shown in U.S. Pat. No. 3,779,094 wherein the nut body incorporates a series of thread-engaging roller elements. One is fixed against axial or translational movement on the nut body, while others are capable of undergoing shifting movement respectively between oppositely disposed extreme positions. All the rollers are carried by needle bearings, with anti-friction thrust bearings and bowed spring washers for biasing the rollers into engagement with the screw threads. The axially shiftable rollers are carried in slide grooves, arranged parallel to the nut axis. Virtually all friction which was encountered was of the rolling-type as opposed to the sliding type. As a result, very high efficiency and smooth operation were obtainable.
One of the problems associated with the automatically reversing nut device disclosed in the above identified patent is that the use of multiple needle bearings and thrust-type ball bearings tended to increase the overall cost of the device. In addition, the adjustment of such bearing assemblages tended to be both critical and time-comsuming. Special machining was required on the screw in order to precisely form the grooves thereof in a way to accept the conical-shaped nose portions of the roller elements. Also, the slides on which the shiftable rollers were carried represented additional parts which had to be machined to close tolerances in order to insure smooth, trouble-free operation and performance.
Also, due to slight variations in the thread pitch between adjacent areas of the screw, there tended to be a degree of looseness between the latter and the rollers. This gave rise to backlash, which was undesirable for certain more critical applications where fairly close tracking of the nut on the screw was required.